Thyratron waveguide switch with density enhancement for operation in 27 to 40 ghz. range



Nov. 25, 1969 H. GQLDIE 3,480,828

THYRATBN WAVEGUIDE SWITCH WITH DENSITY ENHANCEMENT FOR OPERATION IN 27 TO 40g HRANGE Filed Feb. 13, 1967 INVENTOR Horry Goldie www@ ATTORNEY United States Patent 3,480,828 THYRATRON WAVEGUIDE SWITCH WITH DENSITY ENHANCEMENT FOR OPERATION IN 27 T0 40 gHz. RANGE h Harry Goldie, Randallstown, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 13, 1967, Ser. No. 615,506 Int. Cl. H01j 19/80, 7/46 U.S. Cl. 315-39 6 Claims ABSTRACT OF THE DISCLOSURE A thyratron waveguide switch having a frusto-conical shaped shield between the cathode and anode to concentrate the electrons into a very high density electron stream capable of producing a plasma barrier which is an effective reflective vattenuator of microwaves without requiring excessive input power to the switch.

In applicants copending patent application Ser. No. 383,639, filed July 20, 1964, there is described a thyratron waveguide switch, sometimes also called a triggered waveguide switch, and also known as a TWS, which is a thyratron type gaseous triode device in which la section of a perforated waveguide, adapted to be incorporated into a `waveguide transmission line for propagating electromagnetic waveguide energy, is sealed into an envelope which encloses the cathode and anode of the triode. The perforated waveguide section also serves the additional function of the control electrode. The perforated waveguide section is provided with pressure windows which seal the waveguide on either side of the envelope to complete the gas retaining enclosure for all of the electrodes. When a suitable bias is applied between the cathode and the control electrode to cause the triode to conduct the electron current from the cathode to the anode through the perforated walls of the waveguide section develops a plasma across the waveguide in a region between the two pressure Windows in the waveguide and serves as an RF barrier which can be alternately switched on and off by controlling the conduction of the gaseous triode.

This invention relates to a novel and improved thy-ratron waveguide switch which is capable of operating effectively in the 27 to 40 gHZ. range.

As indicated above, thyratron waveguide switches have been known heretofore for operation in the frequency band from 1 to 10 gHz. However, as the frequency of operation increases the isolation capability for a given electron density in the plasma decreases. Accordingly, as the frequency increases it is necessary to increase the plasma density. Heretofore, in the millimeter wave frequency band the power input to maintain the required plasma densities became prohibitive from a practical standpoint. The ability of a plasma to reect microwave energy is a function of the magnitude of the electron density, n, which determines the value of the resonance frequency fp=8980r\/n cycles per second. Since n is proportional to the cathode-anode current, ip, 'a pointis reached where the charge density is high enough to accelerate diffusion losses thus limiting further increase'in electron density, n. The electron density is also proportional to the discharge beam current density. The discharge power input is proportional to the current since the voltage in an arc discharge is relatively constant. The fraction of this discharge input power used in ionizing the gas is quite small due to the inefficiency of the beam as an ionizing agent. Accordingly, the required input power to increase the percentage of ionization is more 3,480,828 Patented Nov. 25, 1969 ICC nearly a function of the current squared and depends upon the collision frequency and type of gas in the triode.

Hence,

Pm-112 (2) Since fpzNn Then Pin-fp4 (3) It is to be noted from the above that the microwave isolation is largely dependent on the ratio tp/f. Thus, to double fp, a sixteen-fold increase in the beam power is required and from a practical standpoint this required input power is prohibitive.

As clearly illustrated in the drawings, the present invention provides an improved TWS in which there is interposed between the cathode and anode a conical shield which concentrates the electrons flowing from the cathode into a very dense electron stream which passes through the plasma-focusing cone and the perforated walls of the `waveguide section serving as the control electrode. This increases the current density, and. importantly, the electron density. The electron density increases the plasma resonant frequency and the microwave isolation in accordance with the relations described above. The mouth-to-throat enhancement ratio of the electron density n can be as large as 20 when the cone is electrically floating and the enhancement may be as great as when employing `weak magnetic elds. The improvement in the electron density can `result in isolation as great as 60 db or a little higher. Laboratory experiments using Langmuir probes have resulted in an electron density multiplication of 16 in this device. This corresponds to a X 4 increase in plasma resonant frequency. .70 db isolation has been achieved at 30 gHz. and 60 db at 40 gHz.

The advantages and objects of the present invention will become readily apparent from the following description when considered in connection with the accompanying drawings in which:

FIGURE 1 is a profile sectional view of the present invention;

FIG. 2 is a plan view of FIGURE 1; and

FIG. 3 is a line diagram illustrating the operation of the present invention.

Referring to the drawings, and FIGURE 1 in particular, the improved thyratron waveguide switch of the present invention includes a gaseous triode having a cathode, anode and a control electrode enclosed in a pressure envelope 10 which includes a section of a rectangular waveguide 11, partitioned off by two pressure windows 12 and 13, an anode terminal seal 14, a glass or ceramic cylinder 16 and a glass diaphragm 17. If desired, the diaphragm 17 may be metallic, it being understood that the electrical terminals protruding through the diaphragm would then have an insulating pressure seal with the diaphragm and then the diaphragm ywould be sealed to the bottom of the glass or ceramic cylinder 16. The enclosing envelope is adapted to maintain a gaseous atmosphere around the cathode 18 and the anode 19` and in that portion of the waveguide section 11 between the pressure windows 12 and 13. The section of microwave guide 11 is adapted to be incorporated into a waveguide transmission line for propagating microwave energy and is provided with suitable flanges 21 and 22 for coupling to ordinary flanged waveguide.

The portion of the waveguide section 11 between the windows 12 and 13 constituting a part of the envelope and has its top and bottom broad `walls perforated as at 23 and 24 to permit the flow of electrons from the cathode 18 to the anode 19. Accordingly, the two perforated broad walls of the waveguide section 11 constitute a dual grid at the same time that the waveguide section 11 also serves as a portion of a waveguide transmission line. The flow of electrons from the cathode 18 to the anode 19 across the opening in the waveguide section 11 will create a high density plasma in the region indicated at 20 which constitutes an RF barrier and thus the device is capable of controlling by reective attenuation, the propagation of microwave energy through the waveguide section, and accordingly, through the waveguide transmission line in which it is incorporated.

The cathode 18 may be of conventional construction and may have a plurality of radial vanes. The device is also provided with a hydrogen reservoir and heater element 31 which is conventional in this type of tube.

It has been pointed `out above that the ability of a plasma to affect microwave ener-gy depends upon the magnitude of the electron density in the region where the plasma is formed. The number of electrons flowing from the cathode to the anode in any such device is proportional to the number of electrons emitted from the cathode 18. The power required to produce plasma electrons is a function of the square 4of the number of cathode electrons. Therefore, as the requirement for an increase in plasma electron density is demanded it is essential to provide an increase in the current per unit area in the region where the plasma is to be formed, namely, in the region 20 across the section of the waveguide section 11 between the perforated sidewalls thereof. Obviously, plasma density can be increased by increasing either the heater power applied to the cathode or by increasing the number of electrons emitted per unit time from the cathode by raising the anode voltage. But as seen above, this is diflicult since it demands a square of the amount of input power required.

In accordance with the present invention, the electron density in the region indicated at 20 where the plasma is to be formed is increased by a frusto-conical funnellike shield 36, its maximum diameter at the lower end being slightly greater than the outer dimension of the annular cathode 18. The minor diameter at the upper end of the shield, which is connected to a cylindrical portion 37, is just sufficient to surround the perforations 23 in the broad walls of the waveguide section 11. This shield is preferably supported from the ceramic cylinder 16, or from the waveguide section 11, but is electrically insulated therefrom so that it will serve as an electrically floating electrode in the electron stream and will acquire a negative charge so that it will not attract electrons from the arc between the cathode 18 and the anode 19. The shield 36 produces a funneling action of the drift electrons in the plasma from the cathode in a manner illustrated in FIG. 3 and increases the electron density in the plasma region 20 between the apertures in the broad walls of the waveguide section 11 many fold. Actual experiments without a magnetic field gave a sixteen-fold increase. Accordingly, the electron density multiplication with consequent increase in the plasma resonant frequency is accomplished without the requirement of prohibitive amounts of power supplied to the discharge in the region 20 between the cathode and the anode. The mouth-to-throat diameter of the shield 36, can be as much as and the ratio of the electron density in the plasma region to the electron density in the vicinity of the cathode 18 will be increased accordingly.

It will be seen from the above description that the thyratron waveguide switch is in effect a dual-grid triggerable hot cathode gas triode in which the dual-grid is formed by the perforated sides of a section of waveguide which may be incorporated into a waveguide transmission line in which it is desired to control the propagation of microwave energy. The device may be operated as a grounded-cathode device in which case the section of microwave guide 11 would be electrically insulated from the other portions of the waveguide transmission line which is conventionally grounded. On the other hand, the switch may be operated in a grounded-grid circuit configuration in which event the waveguide section 11 would be at a common ground potential with the remainder of the waveguide transmission line. A circuit forv operating the device in the grounded grid circuit configuration is described and claimed in applicants copending application Ser. No. 615,507, filed Feb. 13, 1967, in the names of Harry Goldie, H. W. Cooper and L. F. Cooper.

In the operation of the present device, regardless of whether or not the device is operating as a groundedcathode or grounded-grid device, a suitable trigger pulse is applied between the cathode 18 and the dual-grid formed by the perforated walls Of the waveguide 11 to initiate the cathode-to-grid plasma. This cathode-grid discharge plasma having once been formed by the trigger pulse, the plasma will be transferred to the inner grid region, in this instance the region 20 of the waveguide section 11. By virtue of the high static potential impressed between the anode and cathode a current arc forms constituting commutation of the device. The result then is a plasma developed in the central region 20 of the waveguide at densities sufficiently high to retiect nearly all of an incident microwave pulse at millimeter wavelengths while transmitting only a very small fraction. The absorbed power is relatively small since the bulk of the excitation energy is obtained from the arc between the cathode and the anode. The frusto-conical shield 36 serves to concentrate the electron flow at the throat region 37 and therefore reduces the amount of power necessary to make the device effective in the frequency range of from 27 to 40 gHz. without requiring excessive power in the cathode.

Experiments on a millimeter TWS as shown in FIG. 1, using a focusing cone area ratio of AMouth/Afrhroat=157 (4) gave an electron density multiplication factor of 16 representing an increase in plasma resonant frequency by a factor of 4. The electron temperatures were l6,000 K. at the throat and 14,600e K. at the mouth. Isolation varied from 60 to 70 db over a frequency ran-ge of 30 to 40 gHz.

I claim as my invention:

1. A thyratron waveguide switch comprising an electron discharge device having a cathode, anode and a control electrode, said control electrode being in the form of a hollow section of waveguide interposed between said cathode and said anode and having perforated sidewalls to permit the ow of electrons from said cathode to said anode across s-aid waveguide section, said section of waveguide being sealed into an envelope structure encompassing said electrodes and including pressure windows closing said hollow waveguide at points on either side of the perforated portions of said walls to complete said envelepe, an ionizable g-as in said envelope, said cathode constituting an extended source of free electrons, and means for increasing the density of said electrons in said waveguide section relative to the density of said electrons at their source.

2. The combination as set forth in claim 1, in which said means for increasing the density of said electrons includes means for confining the flow of electrons to a region in said waveguide section smaller in cross-sectional area than the cross-sectional area of said source.

3. The combination as set forth in claim 1, in which said means for increasing the electron density includes a focusing shield for concentrating said electrons in a region in said waveguide section that is smaller in the dimension parallel to the axis of said waveguide than the corresponding dimension of said source of electrons.

4. The combination as set forth in claim 1, in which said waveguide section is rectangular in shape.

5 6 5. The combination as set forth in claim 4, in which 2,775,692 12/ 1956 Wilchinsky 315-39 X said perforated Walls are the broadwalls of said section. 3,323,002 5/ 1967 Goldie 315-39 6. The combination as set forth in claim 1, in which said means for increasing the density of electrons in said HERMAN KARL SAALBACH, primary Examiner waveguide section is lan electrically conducting shield of fuiste-conica] configuration 5 SAXFIELD CHATMON, JR., Assistant Examiner References Cited Us. C1 X'R UNITED STATES PATENTS 313 188; 333 13 2,427,089 9/1947 Clifford 333-13 10 

